This invention relates to a method and an apparatus for detecting an end point of a dry cleaning process for removing unnecessary substances deposited in an apparatus for producing a semiconductor device.
An apparatus for producing a semiconductor device has a vacuum chamber in which a process gas is activated by heat or plasma to process a semiconductor substrate. After the semiconductor substrate is processed, unnecessary substances including reaction products and deposits are attached to an inner wall of the vacuum chamber as a contamination. Therefore, the vacuum chamber requires a cleaning process in order to remove the contamination. If an operator opens the vacuum chamber and manually removes the contamination, the temperature and/or the pressure in the vacuum chamber must be changed from processing conditions into normal conditions to allow the entrance of the operator. After completion of the cleaning process, the temperature and the pressure in the vacuum chamber must be recovered into the processing conditions to start a next process. This requires a long time and therefore decreases a productivity.
In view of the above, use is generally made of a cleaning method using a cleaning gas. Specifically, the cleaning gas is introduced into the vacuum chamber and activated by the heat or the plasma. The cleaning gas thus activated reacts with the contamination to thereby remove the contamination.
Traditionally, it is assumed that the cleaning process is completed at a time instant after the lapse of a predetermined cleaning time period which is experientially determined through visual observation by the operator.
In case where the cleaning process is carried out by the use of the plasma, the completion of the cleaning process can be detected by monitoring the change in plasma emission spectrum. Such technique is disclosed, for example, in Japanese Unexamined Patent Publications (JP-A) Nos. S63-005532, S63-014421, S63-089684, and H07-169753. This technique makes use of the fact that the plasma emission spectrum observed during the reaction between the contamination and the cleaning gas is different from that observed after the contamination is removed through the reaction.
Alternatively, the completion of the cleaning process may be detected by monitoring the change in pressure within the vacuum chamber. This technique is disclosed, for example, in Japanese Unexamined Patent Publications (JP-A) Nos. S63-129629, H06-224163, H09-143742, and H11-131211. This technique makes use of the fact that the pressure during the reaction between the contamination and the cleaning gas Is different from that in presence of the cleaning gas alone. Specifically, when the cleaning process is started, the pressure in the vacuum chamber increases because the cleaning gas reacts with the contamination to produce a resultant gas. The pressure continuously increases with the progress of the reaction between the cleaning gas and the contamination and, when the contamination is reduced in amount and the reaction is weakened, decreases to gradually approach a predetermined level. The time instant when the pressure reaches the predetermined level is detected as a cleaning end point.
In Japanese Unexamined Patent Publications (JP-A) Nos. H09-260358 and H11-087248, disclosure is made of a technique of detecting the cleaning end point by the use of a particle counter arranged in a discharge line from the vacuum chamber to monitor the contamination detached from the Inner wall of the vacuum chamber as particles. The particle counter comprises a counter capable of counting the number of particles having a diameter of 0.2 xcexcm or more. Every time when a preselected cleaning time period has lapsed, a nitrogen purging is carried out and the number of particles contained in a discharge gas is counted. When the number of particles is decreased to a preselected level or less, the completion of the cleaning process is judged.
Alternatively, use may be made of techniques utilizing the changes in high-frequency voltage, electric potential of a substrate being processed, and plasma impedance. For example, Japanese Unexamined Patent Publication (JP-A) No. S61-256637 discloses the technique of detecting the end point in response to the change in high-frequency voltage. Japanese Unexamined Patent Publication (JP-A) No. S63-128718 discloses the technique of detecting the end point in response to the change in electric potential of the substrate being processed. Japanese Unexamined Patent Publication (JP-A) No. H07-179641 discloses the technique of detecting the end point in response to the change in plasma impedance.
On the other hand, in a thermal processing apparatus which does not use the plasma, the end point of the cleaning process may be detected by a technique of monitoring the temperature within the vacuum chamber. Such technique is disclosed, for example, in Japanese Unexamined Patent Publications (JP-A) Nos. H8-0306628, H10-055966 and H10-163116. Specifically, reaction heat produced during the reaction between the contamination and the cleaning gas is monitored.
In the above-mentioned conventional techniques, however, it is difficult to accurately detect the end point of the cleaning process. Hereinafter, various problems in the conventional techniques will be described.
In the technique of finishing the cleaning process at the time instant after the lapse of the predetermined cleaning time period experientially determined, incomplete cleaning may often be caused. This is because a cleaning time period actually required before the end point of the cleaning process is variable. For example, the amount of the contamination may be changed in dependence upon process conditions. The cleaning time period for complete cleaning may be changed in dependence upon cleaning conditions. In order to avoid the incomplete cleaning, the predetermined cleaning time period must be prolonged. This results in decrease in operation rate of the apparatus and increase in consumption of the cleaning gas.
In the technique of detecting the end point of the cleaning process by monitoring the change in plasma emission spectrum, plasma emission around the center of the vacuum chamber is monitored. It is difficult to monitor the plasma emission caused by the reaction in the vicinity of the inner wall of the vacuum chamber. Therefore, the end point of the cleaning process is often detected erroneously. In addition, this technique is not applicable to a thermal CVD (Chemical Vapor Deposition) process which does not use the plasma.
The technique of monitoring the change in pressure within the vacuum chamber is disadvantageous in the following respect. The contamination partially reacts with the cleaning gas to produce a resultant gas and partially detached from the inner wall as particles because the adhesion strength is weak. The particles are discharged together with the cleaning gas. Therefore, in dependence upon the amount of the part discharged as the particles, the convergence time of the pressure within the vacuum chamber will be varied. This deteriorates the repeatability in detecting the end point of the cleaning process. As compared with the pressure within the vacuum chamber, the change in pressure is so small that the measurement is difficult.
In the technique of counting the number of particles in the discharge line from the vacuum chamber, it is often necessary to repeat the cleaning process and the counting operation of counting the number of particles during the nitrogen purging a plurality of times. In particular, in order to accurately detect the end point of the cleaning process, the cleaning time period is shortened and the counting operation is frequently carried out. Thus, the detection of the end point with this technique is requires a long time and an increased number of steps.
In the techniques utilizing the changes in high-frequency voltage, electric potential of the substrate being processed, and plasma impedance during the cleaning process using the plasma, the change in strength of such electric signal is so small that the stability or the reliability in detection of the end point of the cleaning process is low.
The technique of monitoring the reaction heat produced during the reaction between the contamination and the cleaning gas is mainly used in the thermal process which does not use the plasma. However, as compared with an atmospheric temperature within the vacuum chamber, the reaction heat is so small that the stability or the reliability in detecting the end point. of the cleaning process is low. Furthermore, the relationship between the end point of the cleaning process and a temperature drop time period required before a desired temperature is reached is not constant. Therefore, it is impossible to detect the end point of the cleaning process with high accuracy and excellent repeatability.
It is therefore an object of this invention to provide a cleaning end point detecting apparatus which is capable of accurately detecting an end point of a cleaning process.
It is another object of this invention to provide a cleaning end point detecting method which is capable of accurately detecting an end point of a cleaning process.
According to this invention, there is provided a cleaning end point detecting apparatus for detecting an end point of a cleaning process in which contamination attached to an inner wall of a reaction chamber is removed by introducing a cleaning gas into the reaction chamber to produce within the reaction chamber a cluster cloud resulting from reacting a part of the contamination with the cleaning gas and to detach the other part of the contamination from the inner wall as detached particles and by discharging the cluster cloud and the detached particles from the reaction chamber together with the cleaning gas, the apparatus comprising:
irradiating means for irradiating a laser beam onto the cluster cloud and the detached particles within the reaction chamber to produce a scattered laser beam scattered by the cluster cloud and the detached particles;
monitoring means for monitoring the scattered laser beam as a two-dimensional image information; and
judging means for judging the end point of the cleaning process on the basis of the two-dimensional image information.
According to this invention, there is also provided a cleaning end point detection method of detecting an end point of a cleaning process in which contamination attached to an inner wall of a reaction chamber is removed by introducing a cleaning gas into the reaction chamber to produce within the reaction chamber a cluster cloud resulting from reacting a part of the contamination with the cleaning gas and to detach the other part of the contamination from the inner wall as detached particles and by discharging the cluster cloud and the detached particles from the reaction chamber together with the cleaning gas, the method comprising the steps of:
irradiating a laser beam onto the cluster cloud and the detached particles within the reaction chamber to produce a scattered laser beam scattered by the cluster cloud and the detached particles;
monitoring the scattered laser beam as a two-dimensional image information; and
judging the end point of the cleaning process on the basis of the two-dimensional image information.
In the cleaning process, a part of the contamination reacts with the cleaning gas to produce a reaction product gas. The reaction product gas reacts in a vapor phase to produce the cluster cloud. As a result of production of the cluster cloud, the other part of the contamination is weakened in adhesion strength to be detached from the inner wall as the detached particles. The detached particles are greater in particle size than the cluster cloud. A test region including the cluster cloud and the detached particles is irradiated by the laser beam to monitor the detached particles and the cluster cloud by a laser beam scattering technique. When the detached particles and the cluster cloud are no longer observed because of exhaustion of the contamination, it is judged that the end point of the cleaning process is reached.